The subject matter disclosed herein generally relates to engine control systems, and more particularly to engine electrical load shed control.
Engine acceleration and resultant aircraft handling qualities are often compromised by a requirement to remain below the engine manufacturer's peak loading specifications for such items as an engine-driven direct current (DC) starter/generator (also referred to simply as a “generator”) which is geared to an accessory gearbox (AGB) of the engine. Accessory loads on the AGB compete against the ability to quickly accelerate the engine to develop power. Accessory loads can include pumps, generators, and the like. There is a tradeoff between the maximum DC generator loads that can be accommodated by the engine versus an acceleration schedule of the engine. Excessive loads on the AGB consume engine surge margin and, if poorly managed, can lead to engine compressor stall and loss of engine power. As a consequence, the engine manufacturer typically assumes that maximum loads on the AGB are present all the time and penalizes the engine acceleration schedule accordingly. In a rotorcraft, slow engine acceleration can lead to poor rotor speed management, which diminishes handling with excessive yaw disturbances and an inability to arrest descent quickly.
Further, turbine helicopter engines are limited in the amount of power that can be extracted from the AGB as driven by the engine compressor (Ng). The starter/generator is mounted to an engine-compressor-driven pad and directly affects the ability of the engine compressor to accelerate during rapid power demand, as occurs in helicopter maneuvering. If the electrical load is not limited, it could cause an engine surge. The engine acceleration schedule is typically designed for a worst case peak electrical load, assuming the worst case could occur when the engine is least able to tolerate it, such as maximum acceleration from a low-power condition. As such, both the amount of allowable electrical load and the maximum engine acceleration rate are conservatively constrained. This has an undesirable result of limiting system electrical capacity and limiting aircraft maneuvering performance, i.e., handling qualities.